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Phenobarbital concentration in serum/plasma (quantitative measurement)

Published: August 9, 2026Updated: August 16, 2026

This test measures the amount of phenobarbital, a medication used to control seizures, in your blood. It helps doctors check if your dose is working properly.

Quick summary

["Phenobarbital levels help ensure your seizure control is safe and effective."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
73208-1
Short code
Phenobarb ind plt IgM SerPl Ql FC
Synonyms
Phenobarb ind plt IgM SerPl Ql FC
Category
hematology
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
flow-cytometry
CPT
82784
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

Why This Test Is Done

Your doctor may order this test if you’re taking phenobarbital to manage epilepsy or other seizure disorders. It helps monitor whether your medication is at an appropriate level to prevent seizures without causing excessive sedation or other side effects.

What to Expect Before the Test

No special preparation is needed. You’ll provide a blood sample, usually drawn from a vein in your arm. The process takes a few minutes, and you may feel slight discomfort, but it’s generally quick and painless.

What the Results Mean

Results are used to adjust your medication dose if needed. Low levels might indicate your body is metabolizing the drug too quickly, while high levels could suggest overdose or improper dosing. Your doctor will interpret these results in the context of your overall health and treatment plan.

Possible Side Effects or Risks

While rare, very high phenobarbital levels can cause drowsiness, confusion, or slowed breathing. Long-term use may also affect bone health or increase the risk of infections. Always follow your doctor’s instructions for safe medication use.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Phenobarbital Level

Frequently asked questions

How often should I get this test?
Your doctor will determine the frequency based on your treatment plan, seizure history, and how your body responds to the medication. Regular monitoring helps balance effectiveness and safety.
Can I stop taking phenobarbital without checking my levels first?
No, abruptly stopping phenobarbital can lead to seizures or withdrawal symptoms. Always consult your doctor before making changes to your medication.
What if my test results are outside the expected range?
If results are unusually high or low, your doctor will review them with you and may adjust your dose, recommend lifestyle changes, or investigate other factors affecting your medication levels.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy